Coiled tubing injectors have been used for years to inject coiled tubing into a well, while a drawworks and/or a top drive are conventionally used for raising and lowering threaded tubulars into a well. In either case, weight on bit circuits have been devised to control deploying the coiled tubing string or the threaded tubular string into the wellbore such that the rate of penetration is determined by the weight-on-bit (WOB) rather than a fixed rate of penetration.
A conventional top drive feed system typically uses hydraulic cylinders coupled to the top drive (TD) with a cable, chains, rack and pinion gears, or directly. The cylinders use hydraulic pressure to hold the top drive from traveling downwards, thereby preventing the drill string from descending into the borehole. In this conventional system, the operator may allow the top drive to descend by bleeding off hydraulic pressure to the feed cylinders. This is accomplished by using an operator adjustable relief valve. For example, if the feed system requires 2000 psi to support the drill string, the operator may set the relief valve to 1500 psi. Hydraulic fluid will then bleed off from the cylinder to the hydraulic tank and the drill string descends into the well bore. This pressure differential may result in an exemplary weight on bit of about 20,000 lbs.
The hydraulic system will always maintain this pressure differential. If the drill bit encounters a more difficult formation, the rate of penetration will slow. Conversely if the bit encounters a softer formation, the bit will descend more rapidly. At all times, the pressure differential will be 500 psi and the WOB will still be about 20,000 lbs.
The top drive (TD) travel may be governed by electrical motor, hydraulic 'I5 motor, or mechanical means using a drawworks winch. In these cases, the system is frequently called an “auto drill,” and consists of a mechanical drawworks lowering the drill string into the well bore. A brake on the drawworks controls decent and modulating the brake automatically accomplishes the same goal. More complex systems use electric or hydraulic drive motors on the drawworks controlled with PLC'S (programmable logic controls).
Once a sufficient amount of drill pipe and a bottom hole assembly (BHA) is in the hole at the lower end of the drill pipe, the top drive feed system ceases to push the dill string into the hole. Instead, the desired to “weight on bit” may be obtained by restricting but not′ preventing the drill string from descending into the hole, so that the desired rate achieves the desired weight on bit.
Various techniques have been devised for controlling a drilling system, and particularly a coiled tubing drilling system which utilizes an injector. U.S. Pat. No. 4,585,061 discloses a system for inserting and withdrawing coiled tubing with respect to a well. Another technique for injecting coiled tubing into a well is disclosed in U.S. Pat. Nos. 5,839,514. 6,047,784 discloses the use of a sensor in a coiled tubing system to control weight-on-bit. U.S. Pat. No. 5,291,956 also discloses controlling a coiled tubing injector assembly. U.S. Pat. No. 5,842,530 discloses a telescopic injector frame, and U.S. Pat. No. 6,923,273 discloses different techniques for selectively obtaining a desired weight-on-bit for a tubing system. U.S. Pat. No. 6,536,539 discloses the use of coiled tubing in a horizontal drilling system.
The disadvantages of the prior art are overcome by the present invention, and an improved circuit controller for controlling a coiled tubing injector is hereinafter disclosed.